Start of the Game
Darius will be dangerous because of Hemorrhage and his ability as a Tank, but right at the beginning his strategy will not be complete, which will cause him to be vulnerable or at least not carry out too many solo kills.

Once you get some AD and your Ulti, you will be someone to be reckoned with. To kill with Noxian Guillotone, Darius should know how much damage he does with it so that, once his enemy has a life less than his damage, he will execute it immediately. For this, Hemorrhage is of vital importance. However, Darius's enemies would still not have enough items to deal too much damage or to properly withstand his attacks, so Decimate is of great importance.

Darius should start with a Ravenous Hydra, as his AD will be significantly increased and because his passive triggers the Hemorrhage effect. Afterwards, it would not be bad to think of an Omen of Randuin or a Spirit Face, to get Life, Armor and / or Magic Resistance.
Half of the Game
Darius will need Life, Armor and Magic Resistance to be able to face his enemies efficiently without losing too much life or a not very high percentage. Likewise, Darius could also sacrifice some of his Tank qualities to increase those of a Fighter, such as adding a Bloodthirsty or a Black Blade to his objects in order to cause more damage in less time.

Near this point, the Lines Phase will begin to break down. Take advantage of Trap along with Vicious Strike to be able to lure the enemy towards the team and slow them down to finish them off. Use Noxian Guillotine frequently, but cautiously. If you see that you don't need it, don't use it.

If you're going 1v1 against a champion, use Decimate first to deal bonus damage while keeping your distance. Try zooming in without using Catch and throw a few basics at him. When he starts to escape, use Catch to hold him back and Vicious Strike to make it difficult for him to escape. Use Tithing one last time, and if it still has some life left, finish it off with Noxian Guillotine.
End of the Game
In Team-Fight, Darius can be of very high importance. With enough AD, he will sweep all enemies off the map and with enough Health, Armor and / or Magic Resistance he can take all enemy attacks without being particularly affected.

Because of the Ulti and its set of Skills, making a Pentakill is easier than with other champions: simply, when you and your team have weakened the enemy team enough, you will have to kill them one by one thanks to Noxian Guillotine and its possibility of relaunching it. If you don't kill them all, use your Items or your other Skills to kill the other enemies.